Variable level converter

ABSTRACT

A variable level converter comprising a first variable level converting unit, a second variable level converting unit and an amplifying transistor connected therebetween. Each of the first and second variable level converting units comprises an inverted L-type variable attenuator which includes a variable impedance element. Each variable impedance element is controlled by the same level control signal and has the same electric characteristics.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a variable level converter, and moreparticularly relates to a level adjusting circuit which is preferablyutilized in an AGC (Automatic Gain Control) circuit having a negativefeedback loop therein.

2. The Prior Art

The variable level converter is one of the most important circuit unitsin various kinds of electronically controlled apparatuses. This variablelevel converter is generally comprised of a conventional variableimpedance element, such as an FET (Field Effect Transistor), and a diodeor a transistor. The variable level converter is usually used for,firstly, converting the level of a signal to the other desired signallevel in dependence upon an external level specifying signal, andsecondly for creating the AGC circuit together with an amplifier. In theAGC circuit an output level of the amplifier is supplied, as a levelspecifying signal, to the variable level converter, by way of thefeedback loop, so as to keep the output level at a desired constant.

Such a variable level converter is widely employed in various kinds ofelectronically controlled systems, such as a microwave transmittingsystem, a microwave receiving system or a satellite communicationsystem.

In the prior art, the variable level converter which is comprised ofboth one variable impedance element and the other circuit element, forexample the so-called inverted L-type variable attenuator, has widelybeen known in the world. This variable level converter can convert thelevel of a signal to the other desired signal level; also the variablelevel converter can perform the AGC operation, by utilizing theso-called non-linear characteristics which is created by the variableimpedance element. However, the variable level converter of the priorart has a defect. The defect is that it is difficult to obtain a widerange of level conversion. This is because, when the level is convertedin a wide range, a deleterious non-linear distortion, especially asecond-order non-linear distortion, is produced from the variableimpedance element. In order to maintain a high quality level conversion,such a second-order non-linear distortion must be eliminated from thevariable level converter. Further, it is required, for the variableimpedance element, to have a wide range of impedance value variation soas to obtain a wide range of the level conversion. However, the more theimpedance value varies in a wide range, the more the noise may increase,due to the second-order non-linear distortion.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide avariable level converter which can perform a wide range of levelconversion without inducing the second-order non-linear distortion.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more apparent from the ensuing descriptionwith reference to the accompanying drawings wherein:

FIG. 1 illustrates a circuit diagram of a first embodiment of a variablelevel converter, according to the present invention;

FIG. 2 illustrates a circuit diagram of a P-AGC (Pilot signal AGC)circuit which is constructed by utilizing the variable level converterof FIG. 1;

FIG. 3 illustrates a circuit diagram of a second embodiment of avariable level converter, according to the present invention; and

FIG. 4 illustrates a circuit diagram of a P-AGC circuit which isconstructed by utilizing the variable level converter shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, which illustrates a circuit diagram of a first embodiment ofthe variable level converter according to the present invention, thevariable level converter 100 is mainly comprised of a first variablelevel converting unit 110, a second variable level converting unit 120and an amplifying transistor 130. The first unit 110 is comprised of afixed resistor 111 and a first variable impedance element 112, such asan FET, a diode or a transistor. The second unit 120 is comprised of afirst fixed resistor 121, a second variable impedance element 122, suchas an FET, a diode or a transistor, and a second fixed resistor 123. Thetransistor 130 has both a fixed resistor 131, as an emitter resistorthereof, and a second fixed resistor 123, as a collector resistorthereof. Thus, the transistor 130 forms a grounded-emitter typeamplified; however, a conventional dc biasing circuit for this amplifieris not shown in FIG. 1. An input signal S_(in) is applied to theconverter 100, via an input terminal 140, and then an output signalS_(out) is produced therefrom, via an output terminal 150, under controlof a control signal S_(c) being supplied via a control terminal 160. Thelevel of the input signal S_(in) is converted to the level of the outputsignal S_(out), in accordance with the control information indicatingthe specified value of the level of conversion, which is supplied as thecontrol signal S_(c).

The first unit 110 itself is known as an inverted L-type variableattenuator. Also the second unit 120 itself, except for the resistor123, is known as an inverted L-type variable attenuator. The knowninverted L-type variable attenuator has a defect, that is the levelcannot be varied in a wide range without inducing the second-ordernon-linear distortion, which distortion is produced by the non-linearcharacteristics of the variable impedance element. In contrast, thevariable level converter of the present invention can vary the level tobe converted in a wide range without inducing the second-ordernon-linear distortion, and accordingly the above mentioned defect iseliminated. In order to eliminate the defect, the variable levelconverter satisfies the following five conditions.

(1) The converter 100 includes two independent variable impedanceelements having identical electric characteristics.

(2) The two variable impedance elements are controlled by the samecontrol signal.

(3) The levels of the signals to be applied to the two variableimpedance elements are the same.

(4) The phases of the signals are opposite to each other by 180°.

(5) The transfer functions of the first unit and the second unit are thesame.

The operational principle of the present invention will be clarified bythe following explanations. The transfer function of the first unit 110is expressed by the following equation. That is, ##EQU1## where thesymbol RV₁ represents the resistance value of the first variableimpedance element 112, and the symbol R₁ represents the resistance valueof the resistor 111.

Similarly, the transfer function of the second unit 120 is expressed bythe following equation. That is, ##EQU2## where the symbol RV₂represents the resistance value of the second variable impedance element122, the symbol R₂ represents the resistance value of the resistor 121and the symbol RL represents the resistance value of the resistor 123.Then the total transfer function of the converter 100 is expressed bythe following equation. That is, ##EQU3## where the symbol R_(e)represents the resistance value of the resistor 131. The symbols E_(in)and E_(out) represent the values of the voltages of the input signalS_(in) and the output signal S_(out), respectively. In the aboveequation 3 , the total transfer function f(RV₁, RV₂) is the product ofthe function f(RV₁), that is ##EQU4## a ratio between the resistors 123and 131, that is ##EQU5## and the transfer function f(RV₂), that is##EQU6## The above mentioned ratio ##EQU7## corresponds to the value ofa gain of the amplifier which is comprised of the transistor 130.

With regard to the resistance values RV₁ and RV₂, these values RV₁ andRV₂ should satisfy the following equation 4 , in order to satisfy theaforesaid conditions state in paragraphs (1) and (2).

    RV.sub.1 =RV.sub.2 =RV.sub.c                                4

where the RV_(c) is the resistance value of a variable element, when itis controlled by a nominal control signal. Also, the resistance valuesR₁ and (RL+R₂) should be selected so as to satisfy the followingequation 5 , in order to satisfy the aforesaid condition stated inparagraph (5).

    R.sub.1 =RL+R.sub.2 =R.sub.c                                5

where the R_(c) is a constant. Then the total transfer function f(RV₁,RV₂) of the converter 100 is expressed by the following equation 6##EQU8## As will be understood from the above mentioned equations 1 and6 , the total transfer function f(RV₁, RV₂) is the square of thetransfer function of the single inverted L-type variable attenuator,that is the squared transfer function of the first unit 110, and at thesame time, the total transfer function f(RV₁, RV₂) which has a gain of##EQU9## with respect to the above mentioned squared transfer functionof the first unit 110.

Further, the resistance value R_(e) can be selected to be some value(explained hereinafter), in order to satisfy the aforesaid conditionstated in paragraph (3).

Lastly, the aforesaid condition stated in paragraph (4) can be satisfiedby employing the grounded-emitter amplifier which includes thetransistor 130. This is because the phase of the signal appearing at thecollector of the transistor 130 differs, by 180°, from the phase of thesignal being applied to the base thereof. Consequently, since the levelsof the signals to be applied to the first and second variable impedanceelements 112 and 122 are the same and at the same time the phases ofsaid signals are phase with opposite phase with each other, thesecond-order non-linear distortion is cancelled and cannot be producedfrom the output terminal 150.

The reason why the second-order non-linear distortion cannot be producedfrom the output terminal 150, will be more apparent by referring to thefollowing detailed description. Generally, the output voltage E (SeeFIG. 1) of the first unit 110 is expressed by the following equation 7 .

    E=a.sub.1 E.sub.in +a.sub.2 E.sub.in.sup.2 + . . .          7

Similarly, the output voltage E_(out) (see FiG. 1) of the second unit120 is expressed by the following equation 8 .

    E.sub.out =b.sub.1 (-E)+b.sub.2 (-E).sup.2 + . . .          8

The negative symbols "-" of the voltage E are derived from the fact thatthe phase of the signal appearing at the collector of the transistor 130is opposite to the phase of the signal being applied to the basethereof. In the above equations 7 and 8 , the coefficients a₁ and b₁represent linear factors, such as a transmission gain or loss of thefirst and second variable impedance elements, respectively, also, thecoefficients a₂ and b₂ represent the second-order non-linear distortionfactors. Accordingly, the factors a₁ and b₁ are represented by thefollowing equations 9 and 10 , respectively. ##EQU10## The equation 10may be rearranged to the following equation 11 by substituting theequations 4 and 5 into the above mentioned equation 10 . ##EQU11##

The above mentioned equation 8 may be rearranged to the followingequation 12 by substituting the equation 7 into this equation 8 .##EQU12## Then the second-order non-linear distortion factor is derivedfrom this equation 12 . This factor is represented by the coefficient ofE_(in) 2, that is (a₁ ² b₂ -a₂ b₁). It should be noted that if thefactor (a₁ ² b₂ -a₂ b₁) equals zero, the second-order non-lineardistortion (E_(in) ²) cannot be produced from the converter 100. Thefactor (a₁ ² b₂ -a₂ b₁) can also be expressed by the following equation13 , that is

    a.sub.1.sup.2 b.sub.2 -a.sub.2 b.sub.1 =a.sub.1.sup.2 b.sub.2 -a.sub.1.sup.2 b.sub.2 b.sub.1 =a.sub.1.sup.2 b.sub.2 (1-b.sub.1)  13

by using the fact that the value a₂ equals the value a₁ ² b₂, that is

    a.sub.2 =a.sub.1.sup.2 b.sub.2                              14

This is because the second-order non-linear distortion a₂ equals theproduct of b₂ and the square of the value of the transmission loss, thatis the value a₁.

Therefore, in the above mentioned equation 13 , the factor (a₁ ² b₂ -a₂b₁), or a₁ ² b₂ (1-b₁), is made zero by selecting the value of b₁ to beequal to 1. In other words, from the equation 11 , the value of##EQU13## should be selected as being equal to 1. It should beremembered that in the previously mentioned statement, with reference tothe equation 6 , "the resistance value R_(e) can be selected to be somevalue in order to satisfy the aforesaid condition stated in theparagraph (3)". At this time, said some value of R_(e) is selected to be##EQU14## because the value ##EQU15## should be equal to 1, in order tosuppress the second-order non-linear distortion to zero.

As a result, the transfer function f(RV₁, RV₂) of the converter 100becomes the square of the function f(RV₁) of the single inverted L-typevariable attenuator (see the equation 6 ), and accordingly the levelconversion ratio of the level may be twice the conversion ratio which isobtained by using the single inverted L-type variable attenuator. Forexample, the conversion ratio of such an attenuator is 20 dB, while theconversion ratio of the converter 100 becomes about 40 dB. As previouslymentioned, with reference to the paragraph (3), the levels of thesignals to be applied to the two variable impedance elements 112 and 122must be the same. Such level coincidence is created by the amplifiercomprised of the transistor 130, because the transmission loss in theunit 110 is compensated by the transistor 130. In this case, it ispreferable to create the level coincidence completely, when it isdesired to eliminate the second-order distortion completely. However, ifit is not strictly desired to eliminate the second-order distortioncompletely, the levels of signals applied to the variable impedanceelements 112 and 122 may be slightly different from each other.

The variable level converter 100 may preferably be employed in an AGCcircuit. The AGC circuit which utilizes the converter 100 is illustratedin FIG. 2. The AGC circuit 200 is especially directed to a P-AGC (Pilotsignal AGC) circuit. In FIG. 2, members which have the same referencenumerals or symbols as the those shown in FIG. 1 are identical with eachother. The reference numeral 210 represents an amplifier having a fixedgain. An amplified signal S_(out) from the amplifier 210 is, on the onehand, produced, as an output signal S'_(out), from an output terminal211, and, on the other hand, applied to a feedback circuit 212. Thefeedback circuit 212 comprises a pilot filter 213, a pilot signalamplifier 214, a rectifier 215 and an expander 216. The output signalfrom this feedback circuit 212 is applied, as the control signal S_(c),to the control terminal 160. The pilot filter 213 operates so as toextract the pilot signal from the signal S_(in). The extracted pilotsignal is amplified by the pilot signal amplifier 214, and the amplifiedpilot signal is changed to a dc voltage pilot signal by the rectifier215. The dc voltage pilot signal is converted to the expanded dc voltagepilot signal, by the expander 216, in accordance with an appropriateexpansion characteristic which is suitable for normally controlling boththe variable impedance elements 112 and 122. The operation of theconverter 100 has already been explained with reference to FIG. 1. Thefeedback circuit 212 controls the variable impedance elements 112 and122 so as to increase or decrease the level of the input signal S_(in)when the level of the pilot signal decreases or increases, respectively,due to, for example a fading. Thus, the feedback loop, which is formedbetween the output of the amplifier 210 and the control terminal 160,acts as a negative feedback loop. As already mentioned, the converter100 creates twice the conversion ratio which is obtained by using theinverted L-type variable attenuator such as the first variable levelconverting unit 110. Accordingly, the P-AGC circuit 200 succeeds inproviding twice the automatic-gain-controlled dynamic range which isprovided by a conventional P-AGC circuit comprised of the invertedL-type variable attenuator. At the same time, the P-AGC circuit 200produces no second-order non-linear distortion.

The level conversion ratio can easily be expanded at will by forming amultiple-stage variable level converter, based on the converter 100 ofFIG. 1. In FIG. 3, which illustrates a circuit diagram of a secondembodiment of a variable level converter, the multiple-stage variablelevel converter 300 the first variable level converting unit 110 (seeFIG. 1), "n" second variable level converting units 120-1, 120-2, 120-3. . . 120-n (refer to the second variable level converting unit 120 ofFIG. 1) and "n" amplifying transistors 130-1, 130-2, 130-3 . . . 130-n(refer to the amplifying transistor 130 of FIG. 1). The fixed resistors111 of FIGS. 3 and 1 are identical. The first variable impedanceelements 112 of FIGS. 3 and 1 are identical. Each of the first fixedresistors 121-1, 121-2, 121-3 . . . 121-n of FIG. 3 and the first fixedresistor 121 are identical. Each of second variable impedance elements122-1, 122-2, 122-3 . . . 122-n of FIG. 3 and the second variableimpedance element 122 of FIG. 1 are identical. Each of the second fixedresistors 123-1, 123-2, 123-3 . . . 123-n of FIG. 3 and the second fixedresistor 123 of FIG. 1 are identical. Each of the fixed resistors 131-1,131-2, 131-3 . . . 131-n of FIG. 3 and the fixed resistor 131 of FIG. 1are identical. The reference numerals 340, 350 and 360, respectivelyrepresent an input terminal, an output terminal and a control terminal.Signals S_(in) and S_(c) of FIG. 3 are identical with the signals S_(in)and S_(c) of FIG. 1. The reference symbol S_(OUT) is an output signalwhich is different from the above mentioned output signal S_(out).

When each of the variable level converting units 120-1, 120-2, 120-3 . .. 120-n is comprised of the respective variable impedance elementshaving the same electric characteristics, the following equation 15 isobtained.

    RV.sub.1 =RV.sub.2-1 =RV.sub.2-2 =RV.sub.2-3 . . . RV.sub.2-n =RV.sub.c  15

The meaning of these symbols is similar to the meaning of thecorresponding symbols occuring in equation 4 .

The resistance values of the resistors 111, 121-1 through 121-n and123-l through 123-n should be selected so as to satisfy the followingequation 16 .

    R.sub.1 =RL.sub.1 +R.sub.2-1 =RL.sub.2 +R.sub.2-2 =RL.sub.3 +R.sub.2-3 . . . =RL.sub.n +R.sub.2-n =R.sub.c                            16

The meaning of these symbols is similar to the meaning of thecorresponding symbols occuring in the equation 5 .

Therefore, the total transfer function f(RV₁ . . . RV_(n)) is expressedby the following equation 17 ##EQU16## The meaning of these symbols issimilar to the meaning of the corresponding symbols occuring in equation6 . Since the grounded-emitter type amplifiers in respective variablelevel converting units 120-l through 120-n have identicalcharacteristics, each of the collector resistors (RL) of the transistors130-l through 130-n and each of the emitter resistors (R_(e)) thereofsatisfy the following equations 18 and 19

    RL.sub.1 =RL.sub.2 =RL.sub.3 . . . RL.sub.n =RL.sub.c       18

    R.sub.e1 =R.sub.e2 =R.sub.e3 . . . R.sub.en =R.sub.ec       19

Therefore, the equation 17 can be replaced by the following equation 20. ##EQU17##

As will be understood from equation 20 , the conversion ratio of theconverter 300 becomes (n+1) times the conversion ratio which is obtainedby using the single inverted L-type variable attenuator.

Departing from equation 20 , suppose that each gain ##EQU18## of therespective stages (120-1, 120-2, 120-3 . . . 120-n) is selected so as tosatisfy the following equation 21 . ##EQU19## The symbol RV(0)represents a specified resistance value of each variable impedanceelement when a respective predetermined standard input voltage accordingto the so-called level diagram, is supplied thereto. Consequently, theequation 20 can be rewritten to become the following equation 22 , byutilizing equation 21 . ##EQU20##

As will be understood from equation 22 , the transmission loss createdbetween the input terminal 340 and the output terminal 350 is almostequal to the transission loss created by the first unit 110 (refer tothe equation 1 ). In other words, there is no transmission loss betweenthe units 120-l through 120-n. Accordingly, the above mentioned leveldiagram between the units 120-l through 120-n does not change. As aresult, during the application of the predetermined standard inputvoltage to each unit (120-l through 120-n), the levels of the signalsapplied to the respective variable impedance elements 122-l through122-n are the same, and thereby the previously mentioned conditionstated in paragraph (3) can be satisfied. Also, the phases of thesignals appearing in each two adjacent units (110, 120-1), (120-1,120-2) . . . (120-(n-1), 120-n) are opposite to each other by 180°, andaccordingly the previously mentioned condition stated in paragraph (4)can be satisfied. Thus, the second-order non-linear distortion cannot beprovided from the output terminal 350.

The variable level converter 300 may also preferably be employed in anAGC circuit. The AGC circuit which utilizes the converter 300 isillustrated in FIG. 4. Thus AGC circuit 400 is especially directed tothe P-AGC (Pilot signal AGC) circuit. In FIG. 4, members which have thesame reference numerals or symbols as those shown in FIGS. 2 and 3 areidentical with each other. An amplified signal S_(OUT) from theamplifier 210 is, on the one hand, produced as an output signalS'_(OUT), from an output terminal 411, and, on the other hand, appliedto the feedback circuit 212. The operation of the converter 300 hasalready been explained by referring to FIG. 3. As has already beenmentioned, the converter 300 attains (n+1) times the conversion ratiowhich is obtained by using the inverted L-type variable attenuator, suchas first variable level converting unit 110. Accordingly, the P-AGCcircuit 400 succeeds in providing n times the automatic-gain-controlleddynamic range which will be provided by a conventional P-AGC circuitcomprised of the inverted L-type variable attenuator. At the same time,the P-AGC circuit 400 produces no second- order non-linear distortion,if equation 22 is also satisfied.

As mentioned above, the variable level converter of the presentinvention can provide a very wide dynamic range of level conversion,when compared to the dynamic range of the levels obtained by using theconventional inverted L-type variable attenuator. Further, although thevariable level converter of the present invention provides a very widedynamic range, it produces almost no second-order non-linear distortion,by suitably choosing the circuit elements so as to satisfy the aforesaidequations. Thus, the variable level converter will be very useful forthe AGC circuit employed in microwave transmitting and receiving systemsor a satellite communication system, where the AGC circuit is requiredto attain a very wide range of level conversion without inducingsecond-order non-linear distortion.

What is claimed is:
 1. A variable level converter, for converting aninput signal of a first level to an output signal of a second level,comprising:means for providing a level control signal; a first variablelevel converting unit, operatively connected to said means for providinga level control signal, for receiving the input signal; a circuit stage,operatively connected to said first variable level converting unit andsaid means for providing a level control signal, for producing theoutput signal, said circuit stage comprising a single amplifyingtransistor operatively connected to said first variable level convertingunit and a second variable level converting unit, having an outputoperatively connected to said amplifying transistor, for producing theoutput signal at said output, said first and second variable levelconverting units having the same transfer function; said first variablelevel converting unit comprising:a first resistor operatively connectedto receive the input signal and operatively connected to said amplifyingtransistor; and a first variable impedance element, operativelyconnected to said first resistor, said amplifying transistor and saidmeans for providing said level control signal, so that the impedance ofsaid first vriable impedance element is varied in dependence upon saidsaid level control signal; said second variable level converting unitcomprising:a second resistor operatively connected to said amplifyingtransistor; a third resistor operatively connected to said secondresistor, said amplifying transistor and said output; and a secondvariable impedance element, operatively connected to said thirdresistor, said output and said means for providing said level controlsignal, so that the impedance of said variable impedance element variesin dependence upon said level control signal, said first and secondvariable impedance elements having identical electric characteristics;said means for providing a level control signal including a negativefeedback circuit comprising:a pilot filter, operatively connected to theoutput of said second variable level converting unit, for extracting apilot signal; a pilot signal amplifier operatively connected to saidpilot filter; a rectifier circuit, operatively connected to said pilotsignal amplifier; and an expanded circuit, operatively connected to saidrectifier and operatively connected to said first and second variableimpedance elements, for providing said level control signal.
 2. Avariable level converter, for converting an output signal of a firstlevel to an output signal of a second level, comprising:means forproviding a level control signal; a first variable level convertingunit, operatively connected to said means for providing a level controlsignal, for receiving the input signal; a first circuit stage,operatively connected to said first variable level converting unit andsaid means for providing a level control signal, said first circuitstage comprising a single amplifying transistor operatively connected tosaid first variable level converting unit; and a second variable levelconverting unit having an output operatively connected to saidamplifying transistor, for producing the output signal at said output,said first and second variable level converting units having the sametransfer function; and a plurality of circuit stages operativelyconnected in series to said output of said second variable levelconverting unit, the last of said plurality of circuit stages providingthe output signal; said first variable level converting unitcomprising:a first resistor operatively connected to receive the inputsignal and operatively connected to said amplifying transistor; and afirst variable impedance element, operatively connected to said firstresistor, said amplifying transistor and said means for providing saidlevel control signal, so that the impedance of said first variableimpedance element is varied in dependence upon said level controlsignal; said second varible level converting unit comprising:a secondresistor operatively connected to said amplifying transistor; a thirdresistor operatively connected to said second resistor, said amplifyingtransistor and said output; and a second variable impedance element,operatively connected to said third resistor, said output and said meansfor providing said level control signal, so that the impedance of saidvariable impedance element varies in dependence upon said level controlsignal, said first and second variable impedance elements havingidentical electric characteristics; said means for providing said levelcontrol signal including a negative feedback circuit operativelyconnected to the output of the last of said plurality of circuit stages,for providing said level control signal to said first and secondvariable impedance elements and to each of the variable impedanceelements in said plurality of circuit stages, said negative feedbackcircuit comprising:a pilot filter, operatively connected to the outputof the last of said plurality of circuit stages, for extracting a pilotsignal; a pilot signal amplifier, operatively connected to said pilotfilter; a rectifier circuit operatively connected to said pilot signalamplifier; and an expander circuit, operatively connected to saidrectifier circuit and operatively connected to said first and secondvariable impedance elements and to each of the variable impedanceelements in said plurality of circuit stages, for providing said levelcontrol signal.